It is of extreme importance for maintaining faultless operation of hydraulic driven diaphragm pumps that the allotted quantity of hydraulic fluid is always present in the hydraulic area, that the proper diaphragm motion is ensured and that strain which could lead to damage of the diaphragm is avoided.
It is known from DE-PS 23 33 876 that a leak replenishment device controlled by the diaphragm system can be designed in order to compensate for the hydraulic fluid deficit in the hydraulic area. This means that the diaphragm itself activates the operation of a control valve, whereby a relay valve which is linked with the diaphragm and is attached in a movable manner within the pump body opens a connection in the intake stroke end position from a storage chamber for the hydraulic fluid to the hydraulic area. Leak replenishment can and should only be then carried out if the diaphragm has reached a predetermined boundary position at the end of the intake stroke.
Additional designs of this type of leak replenishment devices for diaphragm pumps are described in DE-PS 28 43 054 as well as in FR-PS 24 92 473.
The control of leak replenishment by the diaphragm system offers numerous advantages in comparison to pressure controlled leak replenishment with a blow valve. In this manner, high suction levels can be overcome on the one hand, with the suction level being limited by the steam pressure of the delivery fluid and hydraulic fluid alone. On the other hand, overloading of the hydraulic area can be avoided, as can occur with pressure controlled leak replenishment through vacuum points. This type of distinctive vacuum points mainly present themselves with large high pressure diaphragm pumps at the beginning of the suction phase if the fluid columns in the suction line are accelerated in a jerky manner upon opening of the upstroke valve. Finally, leak replenishment controlled by the diaphragm system enables hydraulic fluid to be drawn forward upon low differential pressure of less than 0.3 bar as an example, with absolute pressure remaining at approximately 0.7 bar. As a result, gas accumulation in the hydraulic area can largely be avoided which offers corresponding advantages with respect to feed performance and precision. In contrast, pressure controlled leak replenishment requires a relatively high adjustment of the differential pressure at the blow valve to the extent of 0.6 bar, for example, in order to ensure reliable operation. The resulting decrease in pressure in the hydraulic area during the drawing process of 0.4 bar in absolute pressure, for example, leads to increased gas accumulation. This results in decreased feed performance and precision.
It has nonetheless been shown in practice that this known diaphragm pump still has certain weaknesses whose elimination is desirable. For example, before the initial operation of the pump it must be made sure that the diaphragm is in no case displaced too far in the direction of the feed area with respect to the pistons.
Furthermore, only a predetermined amount of hydraulic fluid can be located in the hydraulic area, because too much hydraulic fluid would lead to stress or even the bursting of the diaphragm when the first piston pressure stroke is executed. An incorrect amount of hydraulic fluid in the hydraulic area can nonetheless be expected if during a break in operation a vacuum has appeared at the blow valve or the pressure valve of the feed area. The vacuum present at the blow valve, for example, can reproduce itself through the statically not very thick blow valve in the feed area as well as in the hydraulic area and can lead to the hydraulic fluid being sucked through the piston sealing from the storage chamber to the hydraulic area.
In order to avoid the diaphragm having to be manually repositioned every time before starting the diaphragm pump to prevent damage to the diaphragm, it is already known from DE-OS 41 41 670 that diaphragm stroke limitation can be provided both in the intake stroke as well as the pressure stroke boundary position of the diaphragm. This ensues in the intake stroke boundary position in a purely mechanical manner, namely by means of a support plate against which the diaphragm lies in the intake stroke boundary position. In the pressure stroke boundary position, the diaphragm stroke limitation is however carried out purely hydraulically, in that a valve part, which is located on the piston end of the relay valve of a leak replenishment device, breaks off the hydraulic connection from the piston working area to the diaphragm working area, with the surplus hydraulic oil being forced out through a pressure control valve in the storage chamber.
What it problematic here though is that the hydraulic diaphragm stroke limitation being used is relatively costly and no display device is included which would signal damage or the rupture of the diaphragm.
In order to be able to carry out monitoring of the condition of the diaphragm, it is already known for a diaphragm pump of the same type (DE-OS 40 18 464) to make the diaphragm as a sandwich diaphragm, with the diaphragm consisting of two individual layers held at a distance from each other. The gap between the individual layers is connected with a display device which reacts as soon as the fluid pressure increases upon the break of an individual layer--either from the feed area or from the pressure area--into the diaphragm gap. With regards to this known diaphragm pump, in order to avoid the reciprocal lifting of the individual layers which particularly occurs in the intake stroke, these are connected to numerous locations, particularly through welding, which is nonetheless relatively costly from a technical standpoint and can lead to the splitting of these connections when exposed to high vacuum levels.